The present invention relates to a ground-fault detecting device for detecting a ground fault in high-voltage vehicles such as EVs (electric vehicles) and HEVs (hybrid electric vehicles) and an insulation resistance measuring device for measuring an insulation resistance value at the occurrence of a ground fault.
For example, a ground-fault detecting circuit for an electric vehicle disclosed in JP-B-2933490 is known as a ground-fault detecting device of the above kind. As shown in FIGS. 7 and 8, this ground-fault detecting circuit is to detect a ground fault from batteries B to a vehicle body E in a running drive circuit system A of an electric vehicle. The running drive circuit system A includes the batteries B which are a high-voltage DC power source (e.g., 200 to 300 V), an inverter 2 as a DC-AC converter for converting, into AC voltages, a DC voltage which is supplied from the batteries B via a plus bus 4 as a DC positive-pole supply line and a minus bus 5 as a DC negative-pole supply line, and an AC motor 3 that is supplied with AC voltages from the inverter 2 via a U-phase line 6, a V-phase line 7, and a W-phase line 8 as AC supply lines. The ground-fault detecting circuit includes an oscillation circuit 10 as an AC signal output circuit and a detecting section 20 as a voltage level variation detecting circuit. A coupling capacitor 10A connects a connecting point P of the oscillation circuit 10 and the detecting section 20 and the plus bus 4 of the batteries B of the running drive circuit system A, and interrupts a DC component.
In the oscillation circuit 10, an oscillator 11 generates rectangular pulses having a constant frequency and a duty cycle of 50%. A next-stage impedance converter 12 outputs rectangular pulses having the same duty cycle as the rectangular pulses which are output from the oscillator 11. An AC output signal of the oscillation circuit 10 appears at the connecting point P via a detection resistor 13, which serves, at the occurrence of a ground fault, as part of a voltage divider that is formed by the detection resistor 13 and a ground-fault resistance 41.
The detecting section 20 is provided with a comparator 21 for comparing, with a reference voltage V1, the voltage level at the connecting point P of the detection resistor 13 and the coupling capacitor 10A where an AC output signal of the oscillation circuit 10 appears. The connecting point P is connected to the inverting input terminal of the comparator 21. A reference voltage circuit for setting the reference voltage V1 by divider resistors 22 and 23 is connected to the non-inverting input terminal of the comparator 21.
To protect the impedance converter 12 and the comparator 21 as the operational amplifier from a counter voltage or an overvoltage at the occurrence of a ground fault, protective diodes 15–18 are provided on the output side of the impedance converter 12 and the input side of the comparator 21.
With the above circuit configuration, in an ordinary state that is free of a ground fault, since there is no impedance variation at the connecting point P, rectangular pulses having a greater peak value than the present reference voltage V1 are input to the inverting input terminal of the comparator 21 and hence the comparator 21 outputs rectangular pulses having a duty cycle of 50%. Therefore, a smoothed voltage Vr that is produced by a smoothing circuit 26 consisting of a resistor 24 and a capacitor 25 is lower than a reference voltage V2. Since this smoothed voltage is input to the non-inverting input terminal of a comparator 27, the output of the comparator 27 is at a low level which indicates a normal state.
However, when a ground fault has occurred between the minus bus 5 and the vehicle body E and the ground-fault resistance 41 (see FIG. 7) has appeared, the coupling capacitor 10A is charged to the voltage of the batteries B by the route of batteries B→ coupling capacitor 10A→ detection resistor 13→ impedance converter 12→ ground line GND→ vehicle body E→ ground-fault resistance 41→ batteries B.
At the same time, since the impedance converter 12 outputs rectangular pulses as an AC output voltage, the rectangular pulses are transmitted by the route of detection resistor 13→ coupling capacitor 10A→batteries B→ ground-fault resistance 41→ vehicle body E→ impedance converter 12. Upon completion of the charging of the coupling capacitor 10A, the peak value of the rectangular pulses as the output of the impedance converter 12 decreases to an oscillation amplitude as divided by the detection resistor 13 and the ground-fault resistance 41 and is stabilized at that value.
Therefore, rectangular pulses having a smaller peak value than the reference voltage V1 are input to the inverting input terminal of the comparator 21, whereby the duty cycle of the output of the comparator 21 is changed to 100%. As a result, the smoothed voltage Vr produced by the smoothing circuit 26 consisting of the resistor 24 and the capacitor 25 becomes higher than the reference voltage V2. Since this smoothed voltage is input to the non-inverting input terminal of the comparator 27, the output of the comparator 27 becomes a high level which indicates a ground fault. As described above, when a ground fault has occurred in the batteries B, the ground fault can be detected on the basis of the output logical level of the comparator 27.
However, the related technique described in JP-B-2933490 has the following problems:
(1) The sensitivity of detection of an insulation resistance is low.
(2) The ground-fault detection accuracy is low because a ripple that appears after the smoothing is an error factor.
(3) The resistance to noise on the vehicle side is low.
(4) The accuracy lowers due to a variation in the vehicle-side capacitance.